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Homework Help: Determine volume of HCl added to propionic acid to get pH=1

  1. Aug 4, 2011 #1
    1. The problem statement, all variables and given/known data
    Given 250ml of 0.1M HC3H5O2 (Ka=1.35E-5) What volume of
    a)1.0M HCl is needed to lower the pH to 1.00.
    b) 1.00M NaC3H5O2 raise pH to 4.00


    2. Relevant equations
    pH=pKa-log (HA/A)
    ICE chart

    3. The attempt at a solution
    I used the ice chart for the dissociation of propionic acid. I determined the concentration of H+ and A- to be 1.16E-3. the pH was 2.93. I do not know how to factor in the HCl to determine the volume needed to lower the pH.
     
  2. jcsd
  3. Aug 18, 2011 #2

    Borek

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    Staff: Mentor

    [tex]K_a = \frac {[A^-][H^+]}{[HA]}[/tex]

    rearranged:

    [tex]\frac {[A^-]}{[HA]} = 10^{pH - pK_a}[/tex]

    At pH 1.0 only about 101-4.75=10-3.75=2x10-4 of the acetic acid is dissociated. That means in the first case you can - more or less safely - assume presence of acetic acid doesn't matter much, as the first approximation pH will depend mostly on the HCl dilution.

    Second case is just a buffer solution, use Henderson-Hasselbalch equation.
     
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